N2O flux short-term response to meteorological solicitations and farming practices in a fertilized crop

Using the eddy covariance technique, half-hourly N2O fluxes were measured over a sugar beet crop
(Terrestrial Observatory of Lonzée, BE, ICOS site level 2) between March and October 2016. Several
parameters of data quality control tests were modified to suit the characteristics of N2O. The u* filtering
threshold was determined based on CO2 data as the procedure could not be implemented using N2O fluxes.
The uncertainty on N2O fluxes was assessed for several aspects of data treatment (total random uncertainty,
spectral correction, u* filtering, gap-filling), which were combined to determine the uncertainty on the
budget.
N2O flux variability was characterized by three peak episodes during the experiment, interspersed with
background fluxes. These events were driven by several variables, depending on the time-scale. The more
time had passed after fertilization, the lower the potential for high fluxes was, and by the end of the crop
season, only background flux was recorded. The soil water content at 5 cm was identify as the single trigger
of N2O emission bursts, while intraday oscillations were positively correlated to the variations of surface
temperature. For the first time, an inhibiting effect of surface soil disturbance (seed-bed preparation) on N2O fluxes was observed, which delayed the start of the following emission peak. This observation combined to the
synchronicity between surface temperature and the oscillations of N2O fluxes supports the hypothesis of a
N2O producing microbial community located in the topmost soil layer.
Between fertilization and harvest, the crop emitted 6520 (± 908) μmol N2O m-2 which corresponds to an EF
of 1.3 % - slightly above the IPCC estimate. Our results stress the importance of measuring N2O exchanges in
fertilized crops, as it weighed for 20% of the GHG budget.